1. Field of the Invention
The invention of this application relates to a loose part monitoring method and system for monitoring a loose part in a fluid flow path of a plant facility such as a nuclear installation.
2. Description of Prior Art
One method for monitoring a loose part in a fluid flow path includes: providing a plurality of sensors on a wall of a fluid flow path; analyzing vibration signals detected by the sensors; and generating a predetermined alarm warning that a loose part is present when it is determined that the detected vibration signals are derived from an impact of a loose part.
The accuracy of such an alarm largely depends on whether the detection signals from the sensors are true or false as information to determine the presence of a loose part, in other words, on whether the detection signals are derived from an impact of a loose part or from background noise such as sound of a swirling stream. Therefore, prevention of false alarm to ensure the accuracy of the alarm resides in improving the accuracy of detection signals from the sensors based on which an alarm is generated, in other words, in how false signals derived from background noise can be excluded from the detection signals.
Based on this perspective, various proposals have been made to improve the accuracy of detection signals from sensors (for example, see Japanese Published Unexamined Patent Publication No. JP-A-H01-015696 (Patent Document 1), Japanese Published Unexamined Patent Publication No. JP-A-H01-321397 (Patent Document 2) and Japanese Published Unexamined Patent Publication No. JP-A-H01-214797 (Patent Document 3).
A loose part monitoring system described in Patent Document 1, which focuses particularly on false signals derived from thermal expansion among false signals that may lead to a false alarm and is intended to improve the accuracy of the alarm by excluding the false signals, has a right/wrong determining section and a frequency analyzing section as means for determining whether the detection signals are true or false. The right/wrong determining section “determines, when three alarm signals are received within 0.5 ms, that the detection signals are false signals,” and the frequency analyzing section determines that signals that are generated at long intervals and have low energy are false signals derived from thermal expansion. Then, the false signals are excluded from the subsequent monitoring control.
A loose part monitoring system described in Patent Document 2, which focuses particularly on false signals derived from background noise during start-up and so on among false signals that may lead to a false alarm and is intended to improve the accuracy of the alarm by excluding the false signals, has a right/wrong determining section having a configuration similar to that of the right/wrong determining section described in Patent Document 1 as means for determining whether the detection signals are true or false.
A loose part monitoring system described in Patent Document 3, which focuses particularly on false signals derived from electrical noise and background noise among false signals that may lead to a false alarm and is intended to improve the accuracy of the alarm by excluding the false signals, has a right/wrong determining section having a configuration similar to that of the right/wrong determining section described in Patent Document 1 as means for determining whether the detection signals are true or false.
The loose part monitoring systems described in the Patent Documents 1 to 3 listed above has a right/wrong determining section to discriminate false signals derived from thermal expansion (in the case of Patent Document 1), to discriminate false signals derived from background noise during start-up and so on (in the case of Patent Document 2) and to discriminate false signals derived from electrical noise and background noise (in the case of Patent Document 3) among detection signals from the sensors, and the right/wrong determining section “determines, when three alarm signals are received within 0.5 ms, that the detection signals are false signals.” When such a determination method is adopted, however, there is a possibility that “signals derived from an impact of a loose part” that should be acquired in view of the purpose to monitor a loose part are determined as false signals and excluded erroneously.
Since sound (vibration) travels through steel at a speed of “3 m/ms,” the above “within 0.5 ms” is equivalent to “within 1.5 m” in terms of distance.
Here, a reactor vessel, in general, has four sensors arranged generally symmetrically on a peripheral wall thereof, and therefore the two diagonal lines connecting pairs of sensors opposed to each other with respect to the center of the vessel cross generally at the center of the vessel. Thus, when an impact is generated in a circular area with a diameter of “1.5 m” around the intersection of the two diagonal lines (i.e., the center of the vessel) (which is hereinafter referred to as “particular area”), the sensors theoretically detect signals derived from the impact within a time difference of “0.5 ms”.
Therefore, when it is determined, when three alarm signals are received within 0.5 ms, that the detection signals are false signals as described above, the signals derived from an impact generated by a loose part in the above particular area are erroneously excluded as false signals.
The loose part monitoring system described in Patent Document 1 has a frequency analyzing section in addition to the right/wrong determining section, and the frequency analyzing section determines signals that are generated at long intervals and have low energy as false signals derived from thermal expansion and excludes the signals. The determination method is based on the assumption that abnormal sounds due to an impact of a loose part “are generated at short intervals and have high energy.” Therefore, minor signals that are generated at long intervals and have low energy are excluded as false signals even if they are signals derived from an impact of a loose part.
As described above, the loose part monitoring systems described in the Patent Documents 1 to 3 described above may make a wrong determination about whether the detection signals are true or false, and there is still room for improvement in the reduction of a false alarm.
It is believed that a major reason why the loose part monitoring systems according to Patent Documents 1 to 3 described above may make a wrong determination about whether the detection signals are true or false is that the discrimination between right and false signals, i.e., between impulsive sound (vibration) derived from an impact of a loose part and impulsive sound (vibration) derived from noise, is made based on a secondary or derivative physical phenomenon associated with the occurrence of an impact such as the correlation between the distance between the impact location and sensors and the sound propagation speed or the difference in interval and frequency at which impacts are generated.